[cynicalkane]

Almost every instrument nowdays is tuned in equal temperament. Tuning matters most for old keyboard music, as in, pre-Beethoven.

The reason keys are important is because many instruments have different sound qualities for different notes. Keys close to E minor or G major allow guitarists to get that "twangy" open string sound. You can get a richer sound tuning up, or a thinner "heavy metal" sound tuning down. Keys close to B-flat allow brass players to use more basic tones on their instruments. All singers have certain sound qualities that are only available on certain notes. "Every Breath You Take", for example, would sound different even a semitone off, because Sting's transition from his creepy baritone to his high-pitched whine happens at a very specific part of his range, and the creepy quality of his voice contrasted with the high-pitched pleas for love is one of the most important qualities of the song.

But, yeah, temperament nowdays is almost irrelevant.